Balloon catheters are catheters with inflatable tips which may be used to expand a partially closed or obstructed blood vessel. In such an application it is important that the catheter be as flexible as possible to allow it to travel in small blood vessels, but in prior art catheters the joint between the catheter shaft and the balloon neck tends to be relatively inflexible in the region of the welded joint.
Another problem associated with balloon catheters is that the region of the welded joint tends to swell relative to the rest of the catheter shaft. When polymers, which have their polymer chains oriented in a particular direction, are heated in order to weld two regions together, the melting temperature of the polymer is exceeded, and the polymer chains become randomised. This causes the polymer material to shrink in the direction of orientation of the polymer chains, and at the same time there is a swelling of the material in a perpendicular direction. In some applications, the resultant thickening or swelling of the joint is undesirable. One such application is in the joint between a catheter shaft and a balloon neck in an angioplasty catheter, which, because of the sensitivity of the application should be of a particular and uniform size and thus swelling in the welded area is undesirable.
There are already known in the prior art processes for forming a joint or seam between two regions of plastics material such as by heat-sealing two components of plastics materials.
U.S. Pat. No. 4,251,305 discloses a method of radiant heat-sealing of a balloon onto a catheter in which a length of tubing for forming a catheter balloon is positioned on the thermoplastic shaft of a medical device. Shrink tubing is then placed over each end of the balloon tubing and is pre-shrunk to maintain its position. Radiant heat in a narrow annular band is then applied to the shaft, the balloon tubing and the shrink tubing to seal the balloon tubing onto the shaft.
U.S. Pat. No. 4,025,375 describes a method for continuous welding of sheets into a tube with opposite sides overlapping. The sheet is tensioned longitudinally while it is heated in order to weld the material in order that the thickness of the weld region is more than twice the thickness of the original unheated sheet material.
U.S. Pat. No. 4,990,298 describes a heat shrinkable closure having a central section, which has been hot, stretched and cooled while in the stretched condition sandwiched between a pair of unstretched end sections and in which one end section has a tapered grooved extension. The hot stretching followed by cooling renders that portion of the material heat-shrinkable.
U.S. Pat. No. 4,775,371 describes a method of manufacturing a stiffened dilation catheter. The catheter has a relatively stiff proximal end portion, which is formed by bonding a stiff tubular member coaxially with the proximal end portion of the relatively soft outer tubular member. The distal end portions of the inner and outer tubular members are sealed together to close the distal end of the balloon. A mandrel is inserted into the relatively stiff tubular member, which is then heated and pulled to shrink it down about the mandrel, thereby reducing the wall thickness of the member.
U.S. Pat. No. 3,985,601 discloses a method for producing an inflatable balloon catheter having an inflatable catheter tip attached to a double lumen catheter shaft to form a smooth continuous outer surface at the outer juncture of the tip and shaft. The catheter shaft has an initial outside diameter approximately equal to the outside diameter of the balloon section, and while in a partially cured state, a leading portion of the extruded shaft is stretched to locally reduce its outside diameter. The entire shaft is then cured.
U.S. Pat. No. 5,769,819 discloses a medical catheter comprising a guide wire shaft defining a guide wire lumen. The catheter has a distal tip extension with a reduced outer diameter to which a balloon is attached. The distal tip extension is tapered at one end by shaving, by radio frequency welding or by compressive heat bonding.
U.S. Pat. No. 4,092,193 describes a coupling means for joining substrates such as tubular articles. The tubular articles include an inner member having a fusable agent and an outer member. The substrates to be joined are inserted between the fusable agent and the outer member and the resulting assembly is heated. The heat raises the temperature of the fusable agent enough so that it melts and flows through and/or about the substrates to be joined with the inner and outer members acting to confine the melted fusable agent by defining boundaries between which it can flow. This bonds the two tubular articles together.
EP-B-371 497 discloses a urethra catheter, which is made by attaching a balloon to the end of a catheter shaft, heating the end portions of the balloon to weld them to the shaft and then machining the shaft tube. The front-end portion of the catheter shaft is reduced in diameter and the balloon is welded to the portion of the shaft having a reduced diameter.
EP-A-483 569 discloses a plastic welding apparatus for sealing a first plastic material to a second plastic material using a laser beam.
EP-B-237 192 discloses a method for welding together plastic or plastic covered surfaces with the aid of laser beam heating.
U.S. Pat. No. 5,807,520 discloses a method of balloon formation involving the steps of extruding a segment of thermoplastic material, maintaining the centre portion at a temperature blow the glass transition temperature of a thermoplastic material, drawing the segment to a pre-determined length, in which, after drawing, the wall thickness of the centre portion does not appreciably change, and then expanding the segment in a mould to produce the balloon.
While many processes are known for welding together joints of plastic material as described above, one problem with the prior art process is that the cross-sectional area of the welded region is greater than the cross-sectional area of the adjoining regions. This creates a swollen area in the weld region, which is unsuitable for many applications. No prior art document teaches a process in which two polymeric materials are first heat-sealed together and the joint is then stretched.